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Vulnerability record · CVE-2021-31876 · published 13 May 2021

CVE-2021-31876: Bitcoin incorrect authorization vulnerability

Bitcoin · Bitcoin

Bitcoin Core 0.12.0 through 0.21.1 does not properly implement the replacement policy specified in BIP125, which makes it easier for attackers to trigger a loss of funds, or a denial of service attack against downstream projects such as Lightning network nodes. An unconfirmed child transaction with nSequence = 0xff_ff_ff_ff, spending an unconfirmed parent with nSequence <= 0xff_ff_ff_fd, should be replaceable because there is inherited signaling by the child transaction. However, the actual PreChecks implementation does not enforce this. Instead, mempool rejects the replacement attempt of the unconfirmed child transaction.

6.5 CVSS 3.1 Medium EPSS 1.6% · top 25.1% CWE-863 · Incorrect authorization
6.5CVSS 3.1 base score, v2 6.4
1.6%EPSS exploitation probability, 30 days
NoNot in CISA KEV
1Affected product versions listed by NVD
10References
17 Jun 2026Last modified by NVD

Description

Bitcoin Core 0.12.0 through 0.21.1 does not properly implement the replacement policy specified in BIP125, which makes it easier for attackers to trigger a loss of funds, or a denial of service attack against downstream projects such as Lightning network nodes. An unconfirmed child transaction with nSequence = 0xff_ff_ff_ff, spending an unconfirmed parent with nSequence <= 0xff_ff_ff_fd, should be replaceable because there is inherited signaling by the child transaction. However, the actual PreChecks implementation does not enforce this. Instead, mempool rejects the replacement attempt of the unconfirmed child transaction.

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:L

Affected products

1 vulnerable configurations from NVD's CPE data, grouped by vendor and product.

References

LinkTags
https://bitcoinops.org/en/newsletters/2021/05/12/ Vendor Advisory
https://bitcoinops.org/en/topics/replace-by-fee/ Vendor Advisory
https://en.bitcoin.it/wiki/Common_Vulnerabilities_and_Exposures#CVE-2021-31876 Third Party Advisory
https://github.com/bitcoin/bitcoin Third Party Advisory
https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2021-May/018893.html Mailing ListThird Party Advisory
https://bitcoinops.org/en/newsletters/2021/05/12/ Vendor Advisory
https://bitcoinops.org/en/topics/replace-by-fee/ Vendor Advisory
https://en.bitcoin.it/wiki/Common_Vulnerabilities_and_Exposures#CVE-2021-31876 Third Party Advisory
https://github.com/bitcoin/bitcoin Third Party Advisory
https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2021-May/018893.html Mailing ListThird Party Advisory

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Same products first, then exploited flaws of the same weakness class.

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Source: NIST National Vulnerability Database (record CVE-2021-31876), CISA KEV, FIRST EPSS (scores of 2026-09-26). This page is refreshed as NVD updates the record.